It is well known that vinyl resins may be plasticized or changed from the hard, horny and stiff state to a soft, plastic workable condition by the addition thereto, at elevated temperatures, of certain plasticizers, such as dioctyl phthalate, and the like. These vinyl polymers or resins are referred to as dispersion resins or paste resins and are usually made employing an aqueous emulsion polymerization technique. In some cases, a suspension polymerization process has been used, but emulsion polymerization is preferred.
When the vinyl dispersion resin is mixed or blended with a plasticizer, it is referred to as a "plastisol". By virtue of the flowability of the plastisol, it can be processed into various useful products. For example, the plastisols can be used in making molded products, films, coatings, and the like. Accordingly, the vinyl dispersion resin must be capable of being mixed with the plasticizers easily and uniformly to form low viscosity plastisols which are stable, containing particles of uniform and proper size, and capable of producing films, and like products, of good clarity.
With the customary emulsion polymerization processes, suitable latices have been difficult to obtain since the latices usually contain particles of varying size and are either too fine or too large. Various proposals have heretofore been made to overcome these difficulties but not with the ultimate success desired. For example, the use of various different emulsifiers and catalysts have been proposed. Also, varying the conditions of polymerization has been suggested. However, in most of these cases, too much coagulation occurred with the resulting latex containing too much coagulum or partially agglomerated particles which precipitate reducing the yield. Further, the shelf life of such latices leave much to be desired. It is desirable to have latices which change very little during storage with respect to viscosity and have and maintain good heat stability.
In U.S. Pat. No. 4,076,920, issued Feb. 28, 1978, a process for preparing vinyl dispersion resins is disclosed and claimed which produces polymers having unique properties for certain end uses. However, here, as in other prior art processes, it is necessary to spray dry the latex or polymer emulsion resulting in aggregates of the polymer particles which requires grinding to break them up into a size capable of being used in plastisols. As a result of the crushing and grinding required to reduce the polymer to a size capable of being used in plastisols and, also because of the varying sizes and irregular shapes of the ground fragments, the plastisols produced from them have high viscosities relative to their solids content. Further, the excessive grinding required develops excessive heat which tends to fuse the polymer.
As opposed to "pearl" or suspension polymerization, where the polymer particles are large enough to be filtered, vinyl dispersion resins cannot be recovered from the water emulsions by filtration and tray drying because they go through the filters and also because the small particles pack tightly together during tray drying forming "cake" that even after grinding remains hard to formulate into smooth plastisols making it practically useless for most commercial operations. The particles of polymer to be used in a plastisol should preferably be spherical in shape to present as small a particle surface as possible for minimum solvation. Also, a dispersion of spheres provides the lowest flow viscosity for charging molds, coating and like operations (See U.S. Pat. No. 3,179,646, issued Apr. 20, 1965).
Another problem in making vinyl dispersion resins is the formation of undesirable polymer buildup on the inner surfaces of the reactor. This deposit or buildup of polymer on said reactor surfaces not only interferes with heat transfer, but also decreases productivity and adversely affects polymer quality, such as producing finer particles than desired with the resultant adverse effect on viscosity. Obviously, this polymer buildup must be removed, with its attendant difficulties. It would be most desirous to prevent or substantially eliminate polymer buildup in the first instance.